Energy dispersive x-ray (EDXD) diffraction has been used since approximately 1967 to obtain diffraction patterns from crystalline regions within an object. A conventional apparatus for performing EDXD is depicted in FIG. 1. A source 30 generates a polychromatic incident x-ray beam incorporating x-rays of different wavelengths. The incident beam passes through the test object 34 being examined. As the x-ray beam passes through the test object, diffraction occurs. The diffracted x-rays are scattered over a range of angles with respect to the incident beam. A radioopaque plate with a narrow opening 36 is arranged to pass only a relatively narrow beam 38 of diffracted rays along a preselected diffracted beam path. An energy dispersive x-ray detector 40 of known type is arranged to capture the diffracted x-ray beam passing through opening 36. The detector is used to measure the diffracted beam intensity as a function of photon energy, to provide a spectrum of the diffracted x-rays. An example of such a spectrum is illustrated in FIG. 2. A substantial part of the incident beam passes through the test object without diffraction. Some of the undiffracted x-rays are absorbed by the test object. The remainder forms a transmitted beam 42 exiting from the object.
Diffracted beam 38 represents the result of x-ray diffraction in a small volume element at the intersection of incident beam 32 and diffracted beam 38. Spectra can be obtained for other volume elements by moving test object 34 relative to the incident and diffracted beams, i.e., relative to source 30 and opening 36. Also, EDXD can be made tomographic by adding multiple detectors and appropriate beam stops and acceptance slits, so that each detector receives a diffracted beam from a different location along the incident beam. In this mode of operation, each detector "looks" at a single volume element of the three dimensional object. This permits a more rapid inspection of the entire object.
After the diffraction spectrum is obtained, it can be analyzed to determine the crystal structure of the diffracting object. This process consists of determining the positions of the peaks in the diffraction pattern and converting the positions into "d" spacings (through Bragg's law) corresponding to the spacing between parallel planes of the crystalline material. This is the most common application for non-tomographic methods. For example, the apparatus shown in FIG. 1 can be used to measure d spacings of crystalline materials under pressure. The apparatus is equipped with diamond anvils 44 to apply pressure on the test object. These anvils are substantially transparent to the x-rays used in the procedure. Alternatively, the diffraction pattern (assuming it comes from an unknown material) can be compared to a library of standard diffraction patterns in order to identify the material. This is possible because each crystalline material has a unique diffraction pattern--thus, two materials can be distinguished by differences in their diffraction patterns. The focus of the present disclosure will be on the identification of unknown materials, although many of the techniques presented here can also be applied to other EDXD methods where only the "d" spacings and their intensities are desired.
Significant efforts have been devoted in the art to design of EDXD equipment. For example, The design of apparatus suitable for EDXD is set forth in U.S. Pat. Nos. 5,007,072, 5,008,911, 5,231,652 5,265,144 and 5,394,453 in German Published Patent Applications 3682453, 3842146, 4203354, 4222227, 3832146, 3909147, 4019613, 4034602 and 4101544 in European Published Patent Applications 360347, 370347, 462658, and 496454; and in scientific literature including Nucl. Inst. Meth. In Phys. Res. A280 (1989) 517-528, Phys. Med. Biol. 35 (1990) 33-41, SPIE, 2511 (1995) 64-70 and International Conference Proceedings, Nuclear Techniques for Analytical and Industrial Applications, Western Kentucky University 1993, pages 171-174. The disclosures of all of these references are incorporated by reference herein.
Considerable effort has been devoted towards application of EDXD methods using such equipment to identification of unknown materials in a test object. In particular, considerable effort has been devoted heretofore towards development of EDXD methods which can detect contraband substances such as explosives and illegal drugs in objects such as parcels and luggage. Practical methods for these applications must meet several demanding conditions. Such a method should be capable of detecting the substances of interest reliably, and should yield only a relatively small number of false indications that a contraband substance is present. The method should be capable of providing such performance despite confounding factors such as variation in the size and x-ray absorptivity of the objects, deliberate attempts to hide the contraband substance by shielding it or by mixing it with other substances. The method also should be usable with real apparatus having imperfections such as x-ray sources with non-uniform emission characteristics. All of these factors taken together pose a significant challenge. Thus, despite all of the efforts in the art heretofore, there has been no truly satisfactory method of EDXD analysis which can be applied in practical detection of contraband substances. There are similar needs for improvement in EDXD methods for other purposes.